1. Field of the Invention
The present invention relates to an active power conditioner and, specifically, to three power electronic switch sets within the active power conditioner, with one of them being switched in high frequency and the other two being switched in low frequency, so as to supply a high quality of power.
2. Description of the Related Art
Owing to the significant improvement of high technology industries, precision equipment is widely demanded in manufacturing processes of high technology industries, and high quality power is required for maintaining a normal operation for the equipment. Besides, the high quality power is also a prerequisite for computer related equipment that is broadly applied. Therefore, how to solve the power quality problems, such as: under voltage, over voltage, voltage spike, voltage distortion, etc., is an important topic for the consumers thereof.
Conventionally, a tap-changer autotransformer is used to solve the problems of under voltage and over voltage. The tap-changer autotransformer electrically connects between an AC power source and a load and provides the load with a stable output voltage by adjusting the taps thereof. However, there are some disadvantages, such as step regulation of the supply voltage, large installation volume, and inability in improvement of voltage distortion.
Recently, active power conditioners, such as the active power conditioner disclosed in U.S. Pat. No. 6,940,188 for example, were developed to overcome the problems of low power quality caused by unstable voltage from a power system. Referring initially to FIG. 1, a schematic circuitry of such active power conditioner is illustrated, which comprises an AC power source 7, a power converter 8, and a load 9. The power converter 8 electrically connects between the AC power source 7 and the load 9, so as to convert the unregulated voltage provided by the AC power source 7 into a regulated high quality voltage for the load 9.
The power converter 8 includes a first power electronic switch set 81, a second power electronic switch set 82, a third power electronic switch set 83, an input filter 84, an output filter 85, and a DC capacitor 86. Each of the power electronic switch sets 81, 82, 83 has two power electronic switches serially connecting with each other. The second power electronic switch set 82 and the third power electronic switch set 83 are jointly operated as a serial-type converter while the second power electronic switch set 82 and the first power electronic switch set 81 are jointly operated as a parallel-type converter. The DC capacitor 86 connects with and between said second and third power electronic switch sets 82, 83 in parallel. The input filter 84 connects between the AC power source 7 and the parallel-type converter. The output filter 85 connects between the serial-type converter and the load 9.
The DC capacitor 86 provides a stable DC voltage for the serial-type converter and the parallel-type converter such that both converters can be operated normally. The serial-type converter serially connects between the AC power source 7 and the load 9 to compensate the AC power source 7 for a voltage variation, so as to provide the load 9 with a stable AC power source. Furthermore, the parallel-type converter connects with the AC power source 7 in parallel, such that the stable DC voltage generated by the DC capacitor 86 is maintained. The input filter 84 and the output filter 85 are employed to filter out the harmonics caused by switching the parallel-type converter and the serial-type converter respectively in high frequency.
Still referring to FIG. 1, when a voltage variation of the AC power source 7 occurs, the serial-type converter converts the DC voltage supplied by the DC capacitor 86 into a AC voltage, so as to compensate the AC power source 7 for the voltage variation to obtain a regulated voltage at the load 9. Therefore, the load 9 may receive a stable AC voltage.
Nevertheless, such an active power conditioner still has some drawbacks in practical applications. For example, when the serial-type converter has to absorb or deliver energy to regulate the voltage applying to the load 9, the energy absorption or delivery can possibly result in a voltage variation of the DC capacitor 86. Consequently, in order to provide the serial-type converter with energy to compensate the AC power source 7 for the voltage variation, the parallel-type converter compensates the DC capacitor 86 for its DC voltage variation by charging or discharging the DC capacitor 86 through the AC power source 7.
Overall, after converting an AC power of the AC power source 7 into a DC power via the parallel-type converter and establishing the stable DC voltage across the DC capacitor 86, the serial-type converter produces a compensating voltage to serially inject the load 9 with said compensating voltage and the voltage of the AC power source 7. As a result, a high-capacitance DC capacitor has to be selected as the DC capacitor 86 for establishing the stable DC voltage. However, due to the high-capacitance of the DC capacitor 86, cost and volume for installation will increase, and reliability will decrease. Furthermore, when the conventional active power conditioner having the circuit topologies of the serial-type converter and parallel-type converter is operated, at least four power electronic switches, that is to say, two power electronic sets are controlled to switch in high frequency. This results in an increase of switching losses. In other words, the power efficiency of the active power conditioner will be reduced. Hence, there is a need in improvement of the conventional active power conditioner.
The present invention proposes an active power conditioner, which can provide a load with a stable voltage when a voltage variation occurs at a AC power source, so that the power quality at the load is improved. Furthermore, the DC capacitor of the conventional active power conditioner is omitted from the active power conditioner of the present invention. Consequently, it has advantages in reduced installation cost and volume and increased reliability. In addition, in operation of the active power conditioner, there is only a power electronic switch set being controlled to switch in high frequency, and the other power electronic switch sets are switched in low frequency. Thereby, the switching losses of the active power conditioner can be reduced, and the power efficiency thereof can also be relatively increased.